Showing first layer RGB weights similarly to AlexNet
It seems Alex has just used the Matlab function mat2gray, as described here: https://www.mathworks.com/help/vision/ug/image-category-classification-using-deep-learning.html
The visual outcome of the features is very similar. mat2gray will simply scale the weights between 0 and 1 (no clipping).
Leaving the (slightly adapted) example code of Mathworks here for future reference:
layer1weights = mat2gray(layer1weights) ;
layer1weights = imresize(layer1weights,5) ; 

figure
montage(layer1weights)
title('First convolutional layer weights')